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Capella

StarDate:

April 27, 2012

At first glance, stellar evolution doesn’t seem to make sense. You might expect the smallest stars to live the longest lives, but that’s not the case. Heavier stars actually live shorter lives than lighter ones.

Consider Capella, the brightest star of Auriga, the charioteer. The golden star is in the west as night falls, well to the upper right of Venus, the “evening star.”

What we see as Capella is actually two stars locked in a tight orbit around each other. The stars were born together -- about 400 million years ago. That makes them less than one-tenth of the age of the Sun. Yet both of Capella’s stars are nearing the ends of their lives.

That’s because both stars are more massive than the Sun. The gravity of heavier stars squeezes their cores more tightly, making them much hotter. Under such extreme heat, the nuclear reactions that power stars take place much more quickly. So even though massive stars have more fuel, they “burn” through it in a hurry.

One of the stars of Capella is about three times the mass of the Sun, while the other is a bit smaller. The heavier star has already “fused” the original hydrogen fuel in its core to make helium, and now appears to be fusing the helium to make carbon and oxygen. That’s caused the star to puff up, forming a giant -- one of the final stages of life. So the star doesn’t have much time left before it can no longer produce nuclear reactions -- bringing its short but brilliant life to a close.

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